Fresh living Bacillus Calmette-Guerin (BeG), injected i.v. into (C57BI/ 6xDBA/2)FI mice, activated peritoneal macrophages rendering them highly cytotoxic for tumor cells in vitro. This cytotoxic activity w s already maximal 14 days after injection of 1 mg of BCG and remained stable when 3 or 5 mg of BCG were given. At the same time spleen cells of the BCG-treat d mice showed strongly depressed responses to the T-cell mitogens, PHA and Con A, irrespective of the dose of BCG injected. The inhibitory effect was shown to be mediated by suppressor cells which had characteristics of macrophages...